1. Field of the Invention
The present invention generally relates to MOS semiconductor devices and, more particularly, to increasing the useful life of a semiconductor device by monitoring and compensating for degradation due to the hot-electron effect.
2. Description of the Related Art
The "hot-electron" effect is a wear out mechanism that degrades metal-oxide semiconductor field effect transistor(MOSFET, or simply MOS) device performance as a function of applied voltage over time. Device threshold voltages and drive currents change with time and eventually cause the device to cease functioning within its design parameters. The hot-electron effect limits the usable applied voltage and therefore hampers device performance, and shortens the usable lifetime at a given applied voltage.
As MOS semiconductor devices become smaller and more highly integrated, degradation from the hot-electron effect has a greater impact due to the proportionately larger voltages and, thus larger fields, experienced by the smaller devices. As a result of these larger fields, some channel carriers (holes or electrons) have enough energy to enter the oxide insulating layer near the drain. For example, in p-type MOSFET devices (PFETs), holes may become trapped in the oxide layer resulting in a positive oxide charge near the drain which reduces the capacity of the channel and degrades device performance. Similarly, in n-type MOSFET devices (NFETs), electrons may become trapped in the oxide layer creating interface traps, eventually leading to gate-to-drain short circuits resulting in device failure.
In order to combat the hot-electron effect several solutions have been devised. Relatively simple solutions involve merely seeking to minimize the damage due to the hot-electron effect, such as by limiting the device channel length. More elaborate solutions involve the so-called lightly doped drain (LDD) wherein a lightly doped extension of the drain is inserted between the channel and the drain. The LDD is designed to spread the drain-to-channel voltage drop as evenly as possible in order to reduce any peaks in the field in order to keep the number of energetic carriers, holes or electrons, available to become trapped in the oxide layer to a minimum. However, the LDD solution increases drain resistance and decreases device gain thereby altering device performance.